This phase II feasibility trial with embedded process evaluation aims to determine the feasibility of conducting an effectiveness trial of the If I Were Jack intervention in post-primary schools in Northern Ireland. The study will 1) assess the acceptability of the intervention to schools (principals and RSE teachers), pupils and parents; 2) identify optimal structures and systems for the delivery of the intervention in the classroom; 3) establish intervention participation rates and reach, including equality of engagement across schools of different socio-economic and religious types; 4) assess trial recruitment and retention rates; 5) ascertain variation in normal RSE practice across the participating schools; 6) develop and refine survey instruments for use in a phase III trial; 7) assess differences in outcomes for male and female pupils; 8) identify potential effect sizes that might be detected in an effectiveness trial and estimate appropriate sample size; and 9) identify the costs of delivering If I were Jack and pilot the methods for economic analysis in a phase III trial.